Generally, in the process of designing a robot of a machine, the weight limit of a motor and the output requirement of a preset torque force are always taken into consideration. Therefore, a motor having smaller power, low torque force, and high rotational speed is usually selected to improve the output torque force together with a harmonic drive (or wave gear drive) having an adequate speed ratio, thereby achieving the output requirement. Compared with common gear transmission structures, the harmonic drive has advantages such as high motion accuracy, nearly zero back clearance, high transmission ratio, light weight, small volume, and large carrying capability. Therefore, the harmonic drive is broadly applied, in addition to being applied in joints of a multi-axis robot, the harmonic drive is also one of the key parts in a feed system of a tool machine and in a fine adjustment mechanism of precise instrument.
The harmonic drive mainly comprises a wave generator, a circular flexible external gear, and a circular rigid internal gear. The wave generator is formed by embedding a deep groove ball thin bearing in an elliptical cam. The circular flexible external gear is an elastic body of a thin cup shape, and external teeth are provided on a surface of a cup rim. The number of the teeth of the circular rigid internal gear is larger than that of the external teeth. During operation, the circular rigid internal gear is fixed, and the elliptical cam rotates due to the input of a high speed shaft. The deep groove ball thin bearing is fixed on the cam, and when the elliptical cam rotates, an external ring of the bearing is forced to be elastically deformed, so as to drive the circular flexible external gear to be elastically deformed to be partially engaged with the circular rigid internal gear. At this point, the circular flexible external gear starts to move with respect to the circular rigid internal gear, and since the numbers of teeth of the circular flexible external gear and the circular rigid internal gear are approximately the same, the circular flexible external gear can output a rather large torque force in a low rotational speed.
During the operation of the harmonic drive, the elastic deformation of the external ring of the bearing and the circular flexible external gear is used to achieve the effect of speed reducing. In the operation, the external ring of the bearing and the balls in the bearing under the driven of the elliptical cam generate rather high frictional heat energy. When the input shaft connected to the harmonic drive rotates in a speed of 3000 r/m, the temperature in the bearing of the wave generator is at least raised to above 75° C., such that the lubricating oil/grease in the bearing may be carbonized due to high temperature. When the lubricating oil/grease is carbonized due to the high temperature, the lubricating effect cannot be achieved, thus causing the damage or failure of the harmonic drive. Therefore, the above problem always limits the rotational speed of the input shaft, thus limiting the application condition of the harmonic drive.
Therefore, how to achieve the requirements on heat dissipation and lubrication of a harmonic drive is a target to be pursued by those researchers.